Photonic crystals are structured optical materials in which the refractive index varies periodically in two or preferably three dimensions. These materials exhibit a range of interesting optical effects when subject to electromagnetic radiation of a wavelength comparable to the spatial modulation of the refractive index. Bragg reflection may occur over a range of wavelengths that depend on the direction of incidence/propagation and the periodicity of refractive index variation. This gives rise to photonic ‘energy gaps’ that are analogous to the electronic band gaps in semiconductors. Typically, electromagnetic waves within a certain frequency range cannot propagate in particular directions within the crystal, and incident electromagnetic radiation at these wavelengths is consequently reflected. It is the presence of such partial photonic band gaps that gives rise to the shimmering colours observed in opal gemstones.
In general there is a complex dependence on the wavelength, direction of propagation and polarisation that dictates which electromagnetic waves may propagate within the photonic crystal and those that are otherwise reflected. However, if the modulation in refractive index is sufficiently strong, propagation of certain frequencies can be forbidden for any crystalline direction, and a complete photonic band gap arises. In this case light is prevented from propagating within the crystal in any direction, and the material acts as an ideal reflector such that all light of a wavelength within the band gap range is perfectly reflected irrespective of the incident direction.
There exist two well-documented methods of fabricating structures with the necessary highly ordered variation in refractive index-microfabrication and self-assembly. Due to the complexity of microfabrication considerable effort has been devoted to investigating self-assembling systems comprised of submicron three-dimensional arrays of dielectric spheres. Such photonic crystals are formed by allowing a colloidal suspension of identically sized spheres to settle slowly under the influence of gravity or by the application of an external force such that the spheres are encouraged to order. One example is the fabrication of synthetic opal structures where uniformly sized sub-micron silica spheres are organised through a sedimentation process into a face-centred cubic crystal structure. Another example is the use of polymer “core-shell” particles. Here a core of a first polymer is surrounded (sometimes with an intermediate layer) by a shell of a second polymer. A photonic crystal material is formed by the heating of the particles such that the shell melts and forms a matrix within which the core particles arrange into a regular structure. This example is of particular interest because of the polymeric nature of the photonic crystal material which provides the potential for a range of new applications.
There is an ongoing desire to improve the techniques used in producing such photonic crystal structures with a view to providing substantial quantities of the materials at a low cost and in a form suitable for later applications. For this reason there is considerable interest in the production of photonic materials as films which may then be incorporated within or applied to a product. Unfortunately a significant problem exists because the fabrication methods inherent within film production are not always amenable to the preservation of the photonic structure of the materials. These effects are further amplified as the thickness of the films is decreased. The result is that film production techniques cause the reduction or even complete loss of any optical effects exhibited by the photonic crystal material. An objective of the invention is to address this problem so as to provide films of photonic crystal material in which a strong optical effect is present.
Further challenges exist in the production of such films on an industrial scale. One such challenge is the production of homogeneous optical properties such that all of the film can be observed to produce a similar optical effect. Known processing techniques can cause variations within the photonic crystal structure as a result of localised strains. These can result in films having significant variations in optical properties in different regions, which increases wastage and costs. Furthermore, other challenges exist in the provision of films which may be readily handled, particularly in the case of films which are thin (for example under 100 micrometres) where the film strength may be low or surface stickiness may prevent ease of use.